Hardcode CLANG_ROOT_PATH
Requiring the including Android.mk file to set CLANG_ROOT_PATH results in
many different values, from external/clang to
frameworks/compile/slang/../../../external/clang. Make does not
evaluate variables inside rules until the rule is run, so all the calls
to transform-*-td-to-out don't get evaluated until the second pass, so
the last value of CLANG_ROOT_PATH set by an Android.mk file is what ends
up in the build rules for all of the modules.
Always set it to external/clang to get consistent results between a top
level make and mm in subdirectory.
Change-Id: Ied6f26929f376fe0643490088994a46b52a33012